The present invention relates to a solid-state image sensor formed of CMOS and a method for fabricating the same.
As a solid-state image sensor using semiconductor materials, an image sensor formed of CMOS (CMOS image sensor) is known.
The CMOS image sensor includes a pixel unit shielded from light, which are called a black pixel unit (also optical black unit) in addition to a light receiving unit of color pixel unit. The black pixel unit is for generating a reference value of a pixel signal in a dark state for the prevention of changes of a signal level due to temperature changes, etc. Accordingly, it is preferable that the black pixel unit is formed isolatable from noises of the surroundings (the color pixel unit and peripheral circuits).
The structure of the conventional solid-state image sensor will be explained with reference to FIGS. 29A and 29B. FIG. 29A is a plan view of the conventional solid-state image sensor, which shows the structure thereof. FIG. 29B is a diagrammatic sectional view of the conventional solid-state image sensor, which shows the structure thereof.
As shown in FIG. 29A, a black pixel array unit 102 is disposed around a color pixel array unit 100 which is a light receiving unit. As shown in FIG. 29B, elements (e.g., photodiodes 104C) forming the color pixel array unit 100 are formed in a p-well 112 formed in a silicon substrate 110. As shown in FIG. 29B, elements (e.g., photodiodes 104B) forming the black pixel array unit 102 are formed in a double well (or a triple well), i.e., a p-well 116 isolated from the silicon substrate 110 by an n-well 114. A shield layer 118 is formed, covering in the color pixel array unit 100 the region other than the regions where the photodiodes are formed, and covering the entire surface of the black pixel array unit 102.
The black pixel array unit 102 is formed in the double well as shown in FIG. 29B, whereby the black pixel array unit 102 can be electrically completely isolated from the color pixel array unit 100 and peripheral circuits (not shown), and the produced images can have high quality.
The solid-state image sensor having the black pixel array unit formed in the double well is described in, e.g., Reference 1 (Japanese published unexamined patent application No. 2002-329854).